Radio communication systems generally include a message transmission device, such as a base station or paging terminal, for encoding messages using a signalling protocol such as FLEX.TM.. The encoded messages are then transmitted as a radio signal to portable devices, such as paging receivers. To increase the probability that a receiving device receives an error-free message, some systems employ "time diversity", e.g., the transmission of duplicate messages.
The receiving devices included in prior art systems are programmed with device addresses for normally receiving messages. Additionally, according to the FLEX.TM. protocol, the receiving devices also store a temporary address that is not used to search for messages until activated. When a group message is to be transmitted to selected devices, a short instruction vector is first transmitted to the selected devices, which are addressed through use of the corresponding device addresses. The short instruction vector activates the temporary address for the addressed devices and directs the devices to a location, e.g., a frame of data, in which the temporary address and a group message associated therewith can be found. The devices then receive the information transmitted in the frame specified by the short instruction vector and recover the group message. When the group message has been received, the devices deactivate the temporary address and once again use only their device addresses for message detection.
Like other messages, the short instruction vector, temporary address, and group message are duplicated one or more times in prior art time diversity systems. However, a device that misses or erroneously receives the initial transmission of the short instruction vector cannot receive the subsequently transmitted group message. Specifically, reception of the duplicate short instruction vector does not enable reception of the group message because the short instruction vector directs the device to a frame of data that was transmitted prior to transmission of the duplicate short instruction vector. So, by the time the duplicate short instruction vector is received, it is too late to receive the frame to which the device is directed by the vector. As a result, missing or erroneously receiving the short instruction vector causes the device to miss the group message.
Thus, what is needed is a method and apparatus for receiving the group message even when an initial transmission of the short instruction vector has been missed or erroneously received.